Rapid heating installations, so-called RTP systems for thermally treating semiconductor wafers are known for semiconductor production. Examples of these installations are described in U.S. Pat. Nos. 5,359,693 and 5,580,830.
In rapid heating installations, semiconductor substrates are subjected to thermal processes in different process gas atmospheres in order to achieve predetermined treatment results. The processing chamber often has a cuboid form in order to ensure even heating of the semiconductor substrates in accordance with the symmetry of the chamber while using bar-shaped heating elements, whereas the objects to be heated are mostly disc-shaped and round, mostly semiconductor wafers, preferably made of silicon, germanium, SiGe, SiC, GaAs or InP.
Thermal processes in rapid heating installations are mostly single disc processes which are generally characterised by rapid heating and cooling procedures. The temperature of the semiconductor wafer should be the same over the whole semiconductor surface at all times during the thermal process. In order to obtain the desired process results, RTP systems must therefore heat up a semiconductor wafer according to a pre-specified temperature/time curve, keep it at a specific temperature for a predetermined period of time (steady state phase), and finally cool it down again according to a temperature/time curve which is often also specified. Spike applications (rapid annealing processes) require particularly rapid heating and cooling procedures. The wafer must be brought to the process temperature (600-1200° C.) within a few seconds and be cooled down again.
With such processes the problem arises that the wafer is heated inhomogeneously during the heating phase during which it is irradiated with very high intensity. This is caused by single and multiple reflections of the primary radiation (radiation which is emitted by the heating elements, mostly halogen and/or arc lamps) on the highly reflective chamber walls which surround the wafer. This causes the effective radiation density of the primary radiation on the edge of the semiconductor disc edge to be greater than in the center of the disc because unlike the reflecting chamber, the center of the semiconductor disc absorbs a much greater part of the radiation than the chamber walls. Consequently the edge of the semiconductor substrate is heated more strongly than the center. The momentary temperature difference between the edge of the wafer and the center of the wafer can easily be 40 degrees Celsius or more.
If the wafer has finally reached its desired temperature, and if it is to be kept at a temperature or cooled down again, the heating elements are correspondingly adjusted so that the radiation of the heated wafer dominates with respect to the primary radiation of the heating elements. During cooling, the lamps are often even totally switched off. Now the opposite effect occurs: due to the radiation of the wafer over its surface, the edge of the wafer cools more rapidly than the center of the wafer. This is because the side chamber limit is further away from the edge of the wafer than the reflectors which reflect the radiation emitted by the center of the disc back to the wafer. As a result, the thermal energy radiated from the outer zones of the wafer close to the wafer edge generally hit the wafer statistically mostly at a flat angle after multiple reflections and dispersion on the reflector walls which are further away. With each reflection energy is lost by absorption on the reflector. In contrast, the radiation of the center of the wafer is reflected directly back onto the center of the wafer by reflectors disposed above and below the semiconductor substrate. The edge regions of the wafer are therefore heated less by the reflected wafer radiation than the center of the wafer when the wafer radiation dominates with respect to the lamp radiation. Furthermore, the surface of the wafer has on its side edge an additional edge surface which also radiates thermal energy. This thermal energy has a low probability of being reflected back to this edge surface after multiple reflections. Consequently the edge of the wafer is now colder than the center of the wafer. Therefore, the temperature/time curve on the edge of the semiconductor disc differs from the temperature/time curve of the center of the semiconductor disc due to these dynamic characteristics.
This problem can be compensated by different means. Early approaches (see e.g. U.S. Pat. No. 5,399,523) provide a processing chamber which is defined by a rhomboid reflector chamber arrangement extending around the processing chamber, the acute angles of which lie above and below the center of the wafer. The processing chamber may also be defined by a cuboid reflector chamber arrangement in which the heating elements and the upper and lower reflectors are disposed a very long way away from the wafer. These embodiments lead to a voluminous reflector chamber arrangement which is not practical in view of space and energy considerations. It has also been proposed to make the chamber walls absorbent so that reflection of the radiation back to the wafer is avoided. However, this requires a much higher radiant power to be emitted by the heating elements which, depending on the process temperature of the wafer, is twice to three times the power required with a reflective chamber arrangement. Consequently, the life of the heating elements is significantly reduced.
In many cases, a ring made of the same semiconductor material as the wafer is placed around the wafer. The ring is spaced only a few millimeters from the wafer. The effect of this is that when heated, the overall semiconductor surface irradiated with the heating elements is artificially enlarged. When using the ring, less additional lamp radiation reflected directly on the chamber walls hits the edge of the wafer and thus a smaller temperature gradient forms between the edge of the wafer and the center of the wafer. On the other hand, this also has an advantageous effect in the steady state and when cooling the wafer because when the lamps are switched off, the ring can emit part of its thermal radiation effectively towards the edge of the wafer material. A disadvantage of this apparatus is that it must be realized using relatively complex mechanical means. The ring for example requires additional support surfaces in the rapid heating system. A further disadvantage occurs predominantly with large wafers (currently 12 inches). When thermally treating such wafers, individual ring segments have to be placed around the wafer because whole wafer rings of the required size are not available industrially, and this further increases the mechanical complexity of the system. Another disadvantage of this apparatus is that the thermal and optical properties of the ring can change over time because, unlike the wafer, it remains in the furnace after the process. This may lead to changes in the thermal homogeneity between the edge of the wafer and the center of the wafer during the thermal treatment thereof, and this can lead to a change in the process results.
Another approach attempts to solve the problem by the heating elements being controlled differently. In this way the thermal gradient between the center of the wafer and the edge of the wafer can be reduced by heating the regions at the edge of the wafer less strongly than the center of the wafer during a heating-up phase, and by heating the edge regions more strongly than the center of the wafer during a steady state phase. This can be achieved, for example by using radiating elements (e.g. spot lamps or rod lamps) above and/or below the wafer which are controlled differently at the edge of the wafer than at the center of the wafer. For reasons of symmetry, when using rod lamps, the wafer must be rotated during the process. An arrangement of rod lamps above and below the wafer with optional additional rod lamps on the side walls of the chamber can, for example, be split into several groups of lamps, one portion of which irradiates the center of the wafer and another portion irradiating the edge regions of the wafer. The additional use of a wafer ring for further reducing the thermal gradient is also conceivable.
Currently the problem of thermal gradient forming between the edge of the wafer and the center of the wafer is solved by a combination of a wafer ring, rotation of the wafer and group control of the (rod-shaped) heating elements. This is technically complex and its use in practice shows that during the steady state phase of a process, with thermal processes which require high temperatures of over 1000 degrees Celsius, the outer lamps must often be operated at their maximum output, whereas the inner banks of lights disposed more above and/or below the center of the wafer contribute to the thermal process with less than half the maximum lamp output in order to compensate for the thermal gradient. This leads to more difficult temperature control. With rapid dynamic processes which only last a few seconds in association with rod-shaped heating elements, the problem occurs that the semiconductor substrate cannot be rotated quickly enough in order to guarantee the required thermal homogeneity over the semiconductor disc during all of the phases of the thermal process.